In electrophotographic image formation, an electrostatic image (latent image) is formed on an electrostatic latent image bearer, and developed with a charged toner conveyed by a developer carrier to form a toner image. The toner image is then transferred onto a recording medium (e.g., paper), and fixed with, for example, heating to obtain an output image. There has been known that the toner remaining on the electrostatic latent image bearer after the toner image is transferred is recovered from the electrostatic latent image bearer by a cleaning member and discharged to a waste toner container.
An image forming apparatus employing a heat-fixing system requires much electricity in the process of heat-melting the toner to be fixed on a recording medium (e.g., paper). Therefore, from the viewpoint of energy saving, low temperature fixing property is one of important properties of the toner. Stable charging property and heat resistant storability are also important in order to continuously output images with a certain level of quality even in a severe use condition, for example, in which temperature or humidity in a use environment of the image forming apparatus varies or in which a large number of images are continuously output.
For the purpose of improving the low temperature fixing property of the toner, it is necessary to control a molecular weight, a molecular weight distribution, and a thermal property of a binder resin that is a main component of the toner. For example, PTL 1 has suggested a toner that contains at least two types of different resins of which softening points are different by 25° C. or more, and chloroform-insoluble components in a range of from 5% by mass through 40% by mass. Each of the resins has a main peak in a range of from 1,000 through 10,000 in a molecular weight distribution of tetrahydrofuran (THF)-soluble components as measured by gel permeation chromatography (GPC). The molecular weight distribution has a half value width of 15,000 or less. The above toner exhibits excellent low temperature fixing property, hot-offset resistance, and heat resistant storability.
However, the binder resin is decreased in the molecular weight, so that low molecular-weight components are increased. The low molecular-weight components contaminate a surface of a charging member or a carrier, or absorb moisture under high humidity. This disadvantageously causes the toner to deteriorate in charging stability.
Meanwhile, for the purpose of improving the charging stability of the toner, the following methods generally have been believed to be effective: a method in which a hydrophobic additive is externally added to surfaces of toner particles to thereby suppress charging reduction of the toner under high humidity, a method in which toner components causing charging reduction of the toner under high humidity are removed, or a method in which contamination resistance of toner components against a charging member or a carrier is improved.
For example, PTL 2 has suggested that low molecular weight components can be prevented from giving an unpleasant odor or contaminating a device by adjusting a content rate of components having a molecular weight in a range of from 500 through 1,000 and derived from a binder resin and a content rate of components having a molecular weight of 500 or less and derived from a binder resin in the toner as measured by GPC. PTL 3 has suggested that low molecular weight components can be prevented from contaminating a developing member by adjusting a ratio of components having a molecular weight of 500 or less in the binder resin as measured by GPC.
However, in the above suggestions, the low molecular-weight components that are effective for the low temperature fixing property are actively removed, leading to a greatly deteriorated low temperature fixing property.
PTL 4 has suggested that both of the low temperature fixing property and storability can be achieved by defining a glass transition temperature of a toner at a predetermined heating rate as measured by DSC, and that the charging stability of a toner can be improved by defining a relationship between an absorbance and a concentration of a solution of the toner in ethyl acetate at a predetermined wavelength, composition of a polyester resin serving as a binder resin, and an acid value and a hydroxyl value of the resin, to thereby decrease amounts of hygroscopic components on surface of toner particles and of hygroscopic components derived from the binder resin.
However, also in this suggestion, the low molecular-weight components that are effective for the low temperature fixing property are removed, so that satisfactory low temperature fixing property is not achieved. Additionally, acid dimers and acid trimers both of which have a molecular weight in a range of from about 1,000 through about 2,000 and both of which deteriorate the charging stability are not taken into account. As a result, satisfactory charging stability has been not achieved.
Thus, there is trade-off among the low temperature fixing property, the heat resistant storability, the charging stability. At present, there has not been achieved a toner being satisfactory in all of the above properties.